![]() ![]() (29−31,33) These investigations suggested Z– E isomerization happening on a picosecond time scale (from A to A′ in Figure 1c). Previous mechanistic studies of DASAs in solution have only focused on the initial photochemical step by means of ultrafast pump–probe spectroscopy and density functional theory (DFT) calculations, in combination with temperature-dependent steady-state UV/vis spectroscopy and photoaccumulation experiments at low temperature. Here, using time-resolved infrared absorption spectroscopy and quantum chemical calculations, we show that competing photoswitching pathways are indeed far more complicated than one would have assumed a priori, and that rational control over it requires “turning knobs” that one normally would not consider. Whereas the actinic step of the reaction has been previously investigated in detail, (29−32) insight into the thermal part of the pathway is as yet largely lacking. ![]() It has become clear ( vide infra) that the functional use of DASAs along a productive photoswitching pathway depends on at least two key steps (see Figure 1c for mechanistic proposal): (23,29) a photoinduced Z– E isomerization within the triene and a thermal electrocyclization. The visible-light-triggered transformation starts from a strongly colored, linear triene (“open”) that cyclizes into a colorless (28) cyclopentenone (“cyclized”, Figure 1a), whose structure depends on the generation of DASAs used (24,25) ( Figure 1b), and then thermally reverts to the original form. The recently introduced donor–acceptor Stenhouse adducts (DASA), (22−27) which have already found a wide range of applications, (27) feature in this respect favorable characteristics. Such switches open novel avenues for tailor-made, user-oriented chemical systems whose functionalities can be manipulated by directing the mechanistic pathway. Going beyond the possibilities offered by these “simple” systems requires photoswitches that undergo addressable transformations along multiple possible reaction pathways. (12−18) Switches such as azobenzenes, (19) stilbenes, hemithioindigos, (20) and diarylethenes (21) rely, for all practical purposes, on a simple transformation that is, the key step for their functioning involves one reaction coordinate such as E– Z isomerization or electrocyclization. (10,11) More recently, they have been used for biological and medicinal applications, with photopharmacology attracting tremendous interest. (1,2) Molecular photoswitches (3) have been particularly successful in this respect as they can be switched reversibly between isomers (4) whose distinct properties can be harnessed in applications ranging from receptors (5) and molecular muscles (6) to machines (7−9) and “smart” materials. Photochemical tools rely on light as external stimulus to manipulate chemical, biological, and materials systems with high spatiotemporal control and without contaminating the sample. These results break new ground for developing user-geared DASA switches but also shed light on the development of novel photoswitches in general. Through this combined experimental–theoretical approach, we are able to unravel the complexity of the multidimensional ground-state potential energy surface explored by the photoswitch and use this knowledge to predict, and subsequently confirm, how DASA switches can be guided along this potential energy surface. The spectroscopic data are interpreted in terms of structural transformations using kinetic modeling and quantum chemical calculations. Here, rapid-scan infrared absorption spectroscopy is used to obtain transient fingerprints of reactions occurring on the ground state potential energy surface after reaching structures generated through light absorption. The remaining thermal switching pathways are to date unknown. ![]() Their photochemistry is well understood, but is only responsible for part of their overall photoswitching mechanism. The recently developed donor–acceptor Stenhouse adducts (DASAs) are versatile switches suitable for such applications. Photoswitches with functionalities that depend on and can be addressed along multiple coordinates would offer novel means to tailor and control their behavior and performance. The operation of commonly employed molecular photoswitches revolves around one key structural coordinate. Switches that can be actively steered by external stimuli along multiple pathways at the molecular level are the basis for next-generation responsive material systems. ![]()
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